1. Field of the Invention
The present invention relates to a process for the condensation of optionally substituted cycloalkanones with optionally substituted anilines with the formation of azomethines in a continuous reaction in a column-like reactor having an applied temperature profile.
2. Description of the Related Art
The condensation of cyclohexanone and aniline to give cyclohexylideneaniline using zinc chloride as catalyst has been known for a long time (Ber. 53 (1920), 345-354). The catalyst system HCl--ZnCl.sub.2 has been employed in such condensation reactions for difficult cases (Ber. 46 (1913), 2718).
The condensation of aromatic amines with cyclohexanones is an equilibrium reaction with a slight heat change. It is shifted in favour of the azomethine (Schiff's base) if the water formed in the reaction is removed from the system. In general, this is achieved by means of azeotropic distillation, if appropriate, employing an inert entraining agent, such as benzene, toluene and the like, for this purpose. Recently the use of TiCl.sub.4 (J. Org. Chem. 32 (1967), 3247) or of molar amounts of (butyl).sub.2 SnCl.sub.2 (Synth. Commun. 12 (1982), 495) has been described for fixing the water of reaction. Both compounds fix the water formed in the reaction with the liberation of HCl.
Other developments have been aimed at fixing the water-fixing species to the aniline by means of a covalent bond, for example in the form of N,N-bis-(trimethylsilyl)-aniline (Bull. Soc. Chim. Fr. 1966, 3205), iminophosphoranes (Angew. Chem. Int. Ed. Eng. 5 (1966), 947) and N-(diphenylaluminium)-aniline (J. Org. Chem. 51 (1986), 1848).
Another method of fixing the water formed in the reaction effectively and thus enabling the reaction to be carried out under mild conditions with a low excess of aniline or ketone, is the use of molecular sieves (J. Org. Chem. 36 (1971), 1570; German Offenlegungsschrift 2,244,238). The disadvantage of this last-mentioned process is the involved and expensive regeneration of the molecular sieve.
Azeotropic removal of water is thus certainly the process of the greatest industrial interest, if it is possible to obtain acceptable reaction times using a small excess of one of the two components, without having to employ major amounts of azeotropic entraining agent.
The condensation of N-phenyl-p-phenylenediamine with cyclohexanone without the addition of a catalyst is described in German Auslegeschrift 1,078,119; in this case it is necessary to employ an excess of cyclohexanone of 200 to 300%.
It is shown in German Offenlegungsschrift 2,525,295 that the reaction time in the condensation of aniline with a 400% excess of cyclohexanone in the absence of a catalyst increases greatly as the batch size increases, so that scaling up to an industrial scale is not possible. It has also been shown that strongly acid and weakly acid organic resins have a favourable effect on the reaction time.
In German Offenlegungsschrift 2,901,863 freshly synthesised, anhydrous, non-calcined calcium hydrogenphophate, apatite of the formula Ca.sub.5 (PO.sub.4).sub.3 OH, dried, noncalcined aluminium oxide-hydroxides and proton-exchanged, aluminium silicates of the montmorillonite type which have been washed until neutral are described as effective catalysts for the reaction of aromatic amines with ketones. The examples in this patent application are limited, however, to the condensation of p-phenylenediamine, which is reactive, with methyl isobutyl ketone, a 150% excess of ketone being used.